The most common question people ask about geocoins is when can I sell a geocoin?
The short and terse answer is: anytime
However, there are some ethical and morals issues that might get you some nasty emails. Geocoin collectors are a close knit community and selling coins that were not meant for sales can exclude you from the group.

Rules of Thumb:
(1) if a coin is commercial (initially sold) whether this is a geocoin sold by a group, an individual or a vendor then you are free to re-sell it.
(2) some geocoins are only given as gifts or traded and the original owner would prefer that these not be sold. It is your job as the seller to find out whether you want to follow the original owner's wishes. There is nothing forcing you to, but it stands you in good steed with other buyers and sellers if you do your homework. In addition, selling a coin which doesn’t belong to you is generally frowned upon unless you have received express permission from its owner, activated coins can be delisted from the site (greatly reducing their collectible value).
(3) a subset of gift geocoins are called mystery coins ~ these are usually altruistic projects where a geocacher will make a coin and gift them out anonymously to other geocachers or place them in geocaches for the next finder to keep. It is generally frowned upon to sell these without permission.
(4) The geocaching website runs and interested geocoin page: http://www.geocaching.com/track/geocoin.aspx